The scan blindness phenomenon [1-6] is a fundamental limitation of microstrip phased arrays. Its elimination is essential to sustain communication links between wide scan angle wireless systems. Even though it is a well-known problem attributed to the coherent coupling between substrate modes and array radiation, its elimination has remained a challenge. In fact, several different approaches have been utilized to completely eliminate it or improve array scan characteristics, each of these approaches has certain limitations on array performance. These limitations include use of: subarrays of elements to distort coherent coupling [7], active devices with parasitic patches and shorting metallic post loaded substrates [8-9], inhomogeneous substrates [10], reduced surface wave antenna elements [11], metamaterials [12-13], defected ground structures [14-15], substrate integrated waveguides [16], and electromagnetic bandgap (“EBG”) materials [17-23]. Further, designs for increasing the scan volume of the arrays by modifying feeding structures or elements [24-27], using novel array elements [28], and taking advantage of excited substrate modes [29] have been utilized. Among these approaches, EBG materials improved scan characteristics the most. In fact, other techniques may improve scan range or partially eliminate blindness along certain directions, but use of EBG materials [30-32] provides omnidirectional frequency bands where surface waves are eliminated along all directions. They are thus the state of the art materials for integrated phased arrays.
Integration of microstrip phased arrays and EBG materials for scan blindness elimination have been avoided because of the coupling of the array and bandgap metallization for performance protection of the isolated structures. Bandgap materials and array elements were characterized separately first, then integrated. For example, in [19-20] authors placed EBG cells away from the array elements to reduce coupling between them. EBG cells surrounded by lossy materials have also been used to suppress higher order modes due to interactions between EBG and antenna elements [23]. Microstrip dipole antenna elements, on the other hand, have been directly printed between EBG cells without devoting room to the radiating elements [18]. Element spacing of scanning arrays is typically of a half wavelength or smaller at the upper edge of the operating frequency band in order to avoid grating lobes in the visible scan ranges. Conventional EBG techniques to remedy scan blindness via said element spacing make the surface of the integrated array structure a metallic cover. These techniques also degrade radiation characteristics of the antenna elements by using lossy materials or smaller radiating elements. A complete solution, for integrating EBG materials and planar phased arrays, that removes bandgap metallization from the radiator layer (to eliminate antenna element size constraints), improves radiation efficiency, and beneficially uses EBG/antenna element interactions has remained a challenge. Artificial magnetic conductors (“AMC”) have been effectively used for suppressing surface waves while projecting AMC behavior in space [33]. Projecting AMC behavior in space is an efficient way to take into account interactions between artificial substrate metallization and antenna elements while improving performance. AMC structures, co-designed with radiating elements, are good candidates for antennas [34] and broadside arrays [33]. Similarly, superstrate materials, designed as an integral part of the radiating structures, have also shown the ability to suppress surface waves from broadside arrays [35]. However, substrate wave elimination must be achieved for all scan angles for phased arrays. Therefore, EBG materials are the most efficient choice.
Surface wave effects and their suppression using EBG structures have been well studied in the context of scan blindness. Suppression of leaky waves also propagate within printed array substrates and potentially induce scan blindness and mutual coupling between array elements. Suppression of these waves have not been investigated within the context of mitigating scan blindness in microstrip phased arrays. A true integrated structure must offer elimination capabilities for both types of substrate waves, i.e., surface and leaky waves. Even though such modeless EBG structures have been proposed in [32], their integration with practical scanning microstrip phased arrays has not been investigated.
It is important to note that radiating structures based on conventional bandgap materials [36-37] cannot be applied to eliminating scan blindness, since the radiating mode (used to characterize the antenna radiation) propagates in the substrate and couples elements. Hence, these structures cannot function as isolated radiating elements that form phased arrays with suppressed mutual coupling or scan blindness. Another type of bandgap material, known as modeless EBG [32], is mode-free along the lateral directions. However, this material does not provide radiation along the broadside direction (i.e., the direction perpendicular to the material) and thus cannot be used as an isolated antenna element to form phased arrays.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.